Wheel speed sensors have been used in a variety of vehicle systems including automatic braking systems (ABS). ABS systems use the signals from the wheel speed sensors to regulate vehicle braking, thus making it imperative that a defective sensor be detected as early as possible. If a wheel speed sensor becomes non-operational or generates an erroneous signal, the system must detect such error and proceed to implement a contingency back-up mode of operation. This is especially important in ABS systems due to the safety related nature of their function.
In prior art methods of detection of the operational state of wheel speed sensors, a microprocessor would be used to look at the output of the sensor after processing by an analog-to-digital converter to confirm that the expected outputs were being generated by the sensors based on the operational state of the vehicle. The problem with this approach is that an unacceptable amount of microprocessor overhead is introduced since the microprocessor must allocate a substantial portion of its operation to processing to look at each individual wheel speed sensor. Another prior art method of detection switches the speed sensors into a check mode. The problem is that in addition to four inputs, this method of status detection requires four outputs to check for individual speed sensor operation requiring additional microprocessor capability.
In normal operation, the microprocessor has to process all wheel speed sensors to arrive at a differential wheel speed signal which is indicative of the amount of wheel slip. Separate wheel speed sensor circuits pre-process the outputs from each wheel speed sensor and then transmit the signals to the microprocessor which calculates the difference between two or more of the speed sensors. This processing can be performed in the wheel speed microprocessor or the ABS microprocessor which then controls the brakes to reduce wheel spin or lock-up. In prior art systems, to detect wheel speed sensor abnormalities, all of the wheel speed sensors had to be connected to separate microprocessor outputs and a like number of switching transistors were required for switching the sensors into a test mode. It would be desirable to reduce the number of outputs required for sensor status detection.
Also, it is difficult to determine the appropriate time to run a detection check since the vehicle operational state is unknown. If the vehicle is stopped, no speed signals are generated and if one wheel is locked, no signal will be generated at that wheel even though the speed sensor is fully operational.
Another prior art method of sensor status detection used is a resistance balancing method where the wheel speed sensor circuit looks for balance between the resistance of the wheel speed sensors (which commonly consist of a multiple turn coil and a core). The problem with this approach is that coil resistance can vary with time or from sensor to sensor due to manufacturing variability, thus requiring complicated resistance balancing schemes to adjust for this variation. In addition to this problem, a similar number of microprocessor outputs are required as was also required with the other prior art methods.